Knowledge of the synaptic connexions of supranuclear structures upon abducens motoneurones and internuclear neurones and accessory abducens motoneurones is important for understanding the central organization of motor units and the relationship of this organization to various types of eye movements. The synaptic terminations of specific afferent brain stem nuclei upon abducens and accessory abducens neurones will be studied in the cat by electron microscope autoradiography, providing a definition of the morphology and distribution of synaptic endings derived from each source. In order to relate this central organization to the peripheral organization of muscle units of the extraocular muscles, correlated histochemical and electron microscopic studies of muscle fibers in the cat lateral rectus and retractor bulbi muscles will be undertaken. Studies that relate the phylogenetic changes in the innervation and function of the lateral rectus and accessory lateral rectus muscles in the monkey will be compared to the results of similar studies of the cat lateral rectus and retractor bulbi muscles.